Cooling mechanism for stacked die package and method of manufacturing the same

ABSTRACT

An apparatus for cooling a stacked die package comprises a first die provided above a substrate; a second die above the first die; a cooling fluid in fluid communication with the first die and the second die, the cooling fluid for absorbing thermal energy from the first and the second die; a housing containing the first and second dies, the housing sealing the first and second dies from an environment, wherein the housing further includes a first opening and a second opening, the first and second openings being vertically displaced from one another; a conduit having one end connected to the first opening and the other end connected to the second opening, the conduit allowing the cooling liquid to circulate from the first opening to the second opening; a first temperature sensor being arranged to provide an output that is dependent on a local temperature at the first opening; and a second temperature sensor being arranged to provide an output that is dependent on a local temperature at the second opening, wherein the outputs of the first and second temperature sensors relative to each other are indicative of a level of the cooling fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation-in-Part of U.S. applicationSer. No. 12/878,319, filed Sep. 9, 2010, and claims priority of U.S.Provisional Patent Application Ser. No. 61/418,281, filed on Nov. 30,2010, which are incorporated herein by reference in their entireties.

BACKGROUND

The disclosure relates generally to stacked die packages and, moreparticularly, to cooling mechanisms for stacked die packages.

Recently, three-dimensional integrated circuit (3D IC) packages, orstacked die packages, have provided a possible solution to traditionaltwo-dimensional (2D) ICs in overcoming the interconnect scaling barrierand for improving performance. In stacked die packages, multiple diesare stacked together using vertical through silicon vias (TSVs) wherelonger wire connections and inter-die input/output (I/O) pads areeliminated. The overall performance is significantly improved withfaster and more power efficient inter-core communication across multiplesilicon layers.

As effective as 3D IC technology is, 3D IC technology faces criticalthermal management challenges. When multiple dies are stacked verticallyin a package, the thermal path for dissipating heat generated by thedies is limited. Stacked die packages are typically encapsulated in amaterial that does not dissipate heat well and, if the heat dissipationproblem is not addressed, the dies may overheat during operation leadingto possible problems with transistor performance and reliability. Toaddress the heat dissipation problem, cooling systems that use thermalvia and liquid micro channels have been proposed. However, such systemsare complex and expensive to implement.

BRIEF DESCRIPTION OF DRAWINGS

The features, aspects, and advantages of the disclosure will become morefully apparent from the following detailed description, appended claims,and accompanying drawings in which:

FIG. 1 is a cross-sectional view of a stacked die package according toan embodiment of the present disclosure.

FIG. 2 is a cross-sectional view of a multi-chip system packageaccording to an embodiment of the present disclosure.

FIG. 3 is a cross-sectional view of a different stacked die packageaccording to an embodiment of the present disclosure.

FIG. 4 is a flowchart illustrating a method of manufacturing a stackeddie package having a cooling mechanism according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of embodiments of the presentdisclosure. However, one having an ordinary skill in the art willrecognize that embodiments of the disclosure can be practiced withoutthese specific details. In some instances, well-known structures andprocesses have not been described in detail to avoid unnecessarilyobscuring embodiments of the present disclosure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment.

Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. It shouldbe appreciated that the following figures are not drawn to scale;rather, these figures are merely intended for illustration.

FIG. 1 is a cross-sectional view of a stacked die package 10 accordingto an embodiment of the present invention. Stacked die package 10includes a substrate 20, a first die A, a second die B, a third die C, afourth die D, a housing 40 and a cooling fluid 60 contained in a cavityof housing 40. Substrate 20 may comprise a silicon substrate althoughother semiconductor substrates, such as silicon-germanium substrate,III-V compound substrate, glass substrate, or silicon on insulator (SOI)substrate may be utilized in various embodiments. Dies A, B, C, and Dmay include one of a processor die, memory die (e.g., SRAM, DRAM), powerdevice die, an ASIC (application specific integrated circuit) die, orother functional device dies. Dies A, B, C, and D may comprise aplurality of through silicon vias (TSVs) (not shown) for inter-diecommunication, silicon or other semiconductor materials and may includeone or more conductive layers (not shown). There may be multiplemetallization layers (not shown) formed within dies A, B, C, and D, anddies A, B, C, and D may include a plurality of other layers, such asinter-metal dielectric (IMD) layers (not shown). Dies A, B, C, and D mayalso include other active components or circuits, such as transistors,capacitors, and other devices. Bumps 30 sit on pads (not shown) andprovide electrical connections between the dies.

Although FIG. 1 shows the stacked die package 10 as having four dies A,B, C, and D stacked upon one another, one skilled in the art willunderstand that the stacked die package 10 may have two or more diesstacked one upon the other in some embodiments while in otherembodiments the stacked die package 10 may have more than four dies.

To address the heat dissipation problem in stacked die package 10, anapproach according to an aspect of the present invention is to immersedies A, B, C, and D in a cooling fluid. A volume of cooling fluid 60 iscontained in housing 40 with the housing 40 hermetically sealing dies A,B, C, and D from ambient air or some other environment. The coolingfluid 60 both cools and insulates dies A, B, C, and D. Cooling fluid 60helps cool dies A, B, C, and D by absorbing heat generated by operatingdies A, B, C, and D and drawing the heat away from the dies to the wallsof housing 40 where the heat is then dissipated to the ambient air.

Cooling fluid 60 can comprise a fluid or liquid. As an example, coolingfluid 60 can comprise a fluid, such as oil, dielectric oil, water, amixture of water and an anti-freezing agent, potassium formate,perfluorinate coolant, or the like. As a particular example, the coolingfluid 60 may comprise a non-electrically conductive liquid perfluorinatecoolant, such as those made by 3M™ including 3M's HFE-7100 coolant andsimilar coolants.

In some embodiments, the cooling fluid 60 comprises a two-phase liquid,such as any two-phase liquid commercially available from variousmanufacturers. One skilled in the art will understand that cooling fluid60 may be any fluid capable of absorbing and releasing energy and may bein a fluid form, such as water, gas, oil, or a mixture thereof.

In operation, a volume of cooling fluid 60, such as oil for example,heated by dies A, B, C, and D within housing 40 rises upwardly towardsthe top of housing 40. As the oil rises towards the top of the housing40, upward flow is restricted and lateral flow occurs. Also, as heatedoil cools, its density increases with a resultant downward flow aided bygravity. The downward flow is limited by the bottom of housing 40consequently establishing a lateral flow to again bring the coolingfluid into engagement with the dies to begin the cycle anew. It isunderstood that the level of the cooling fluid should be maintained at aprescribed level as otherwise the temperature of the operating dies maynot be sufficiently lowered.

The housing 40 defines the cooling fluid compartment and contains thecooling fluid 60 therein. The housing 40 has a generally rectangularshape but other shapes are also contemplated, such as a shape or designcapable of placing the cooling fluid 60 and dies A, B, C, and D inefficient heat exchange with one another. Housing 40 may be constructedof a material, such as steel, aluminum, copper, silver, metal, silicon,or silicon carbide. Other materials, such as gold, though perhaps lesscost effective than those already mentioned, are also thermallyconductive to an adequate or even optimal degree and may also be used incertain embodiments.

To assist cooling of dies A, B, C, and D, in some embodiments an outsidesurface of housing 40 includes a plurality of radiators or fins 50 forheat dissipation. Fins 50 may be disposed on any or all of the outsidesurface(s) of housing 40. The fins 50 provide additional surface areafor establishing heat transfer between the heated cooling fluid and theambient air. Fins 50 may be elongated for efficient thermal energytransfer to the ambient air and may be constructed of a material such assteel, aluminum, copper, silver, metal silicon, or silicon carbide. Oneskilled in the art will understand that fins 50 may be made from anymaterial having a relatively high thermal conductivity. Although fins 50as depicted in FIG. 1 are rectangular in shape, such shape is not arequirement, and fins 50 can have a shape that is square, oval,circular, or a variety of other shapes capable of assisting with heatdissipation from stacked die package 10. Fins 50 are affixed to an outersurface of housing 40 by soldering, brazing, bonding, or by some othermanner.

In some embodiments, the stacked die package 10 includes a pressurerelease apparatus 65. For convenience of illustration and ease ofunderstanding, the pressure release apparatus 65 is shown in FIG. 1simply as a box. The pressure release apparatus 65 releases pressure onthe housing 40 caused by cooling fluid 60. When the temperature in thestacked die package 10 increases there is a corresponding increase inthe pressure of the cooling fluid 60. If the pressure of the coolingfluid 60 is not offset, this pressure may rupture the housing 40 of thestacked die package 10. The pressure release apparatus 65 releases thepressure in cooling fluid 60 to prevent such a rupture. As one skilledin the art will understand the workings and construction of a pressurerelease apparatus, the details of such will not be described herein.

In some embodiments, stacked die package 10 includes a deionizer 75 oran apparatus to deionize ions in the cooling fluid 60 that may begenerated by the interaction between the cooling fluid 60 and componentsof the stacked die package 10, such as dies A, B, C, D, or bumps 30. Ifthe cooling fluid 60 is not de-ionized, conductivity of cooling fluid 60may increase causing shorts in one or more dies A, B, C, or D, therebydamaging them. One skilled in the art will appreciate how a deionizer isconstructed and for convenience the details of such will not bedescribed herein.

The teachings of the present disclosure of immersing stacked dies in acooling fluid contained in a housing can also be applied to a multiplechip package. FIG. 2 is a cross-sectional view of a multi-chip systempackage 15 according to an embodiment of the invention. The multi-chipsystem package 15 may comprise many different chips, stacked chips, andcomponents, such as 3D IC packages, MEMs packaging, system on chips(SOCs), THERMAL SOPs, OPTO SOPs, embedded components, antennas andfilters, and the like. A volume of cooling fluid 60 is contained inhousing 40. Cooling fluid 60 both cools and insulates the components ofmulti-chip system package 15. The cooling fluid 60 helps cool thecomponents by absorbing heat generated by the components and drawing theabsorbed heat to the walls of housing 40 where the absorbed heat is thendissipated to the ambient air. In some embodiments, an outside surfaceof housing 40 includes a plurality of radiators or fins 50 foradditional heat dissipation.

Although cooling fluid circulation within housing 40 may be achieved bypassive means as described above, in another embodiment of the presentinvention an active pumping action with the use of a mechanical pump 80is employed to circulate the cooling fluid 60. FIG. 3 depicts thestacked die package 10 of FIG. 1 having a pump 80 and a conduit 85. FIG.3 does not depict a pressure release apparatus 65 for ease ofillustration. One end of the conduit 85 is connected to a lower inlet oropening in housing 40 and the other end of the conduit 85 is connectedto an upper outlet or opening in housing 40. The conduit 85 can be apipe, tube or any suitable passageway for allowing cooling fluid 60 tocirculate from the upper opening to the lower opening. Pump 80 iscoupled to conduit 85 for pumping the cooling fluid 60 from the upperopening to the lower opening of housing 40. Pump 80 can be any apparatusfor circulating the cooling fluid by means of a piston, plunger, or aset of rotating vanes, for example. In operation, the pump 80 pumpscooling fluid 60 from a bottom region of housing 40 where the coolingfluid is generally cooler to an upper region of housing 40 where thecooling fluid is generally warmer as compared to the cooling fluid atthe bottom region. The circulation of cooling fluid 60 from the lowerregion to the upper region of housing 40 cools dies A, B, C, and D.

The teachings of the present disclosure of employing active pumping forcirculating fluid in housing 40 can be equally applied to the multi-chipsystem package of FIG. 2. It is to be understood that cooling fluid flowcontained within housing 40 in these embodiments and others may becirculated by one or more methods, such as by gravity, an active pumpingaction, such as with the mechanical pump described above, a passivepumping action, such as with a wicking action, a thermal siphoningaction, or the like.

Still referring to FIG. 3, in some embodiments stacked die package 10includes one or more barriers 96 disposed within the housing 40 of thestacked die package 10. Barriers 96 help direct the fluid flow F ofcooling fluid 60, particularly to areas between two stacked dies, in theregion of the bumps 30. Without the barriers 96, a substantial amount ofcooling fluid 60 may flow over the top of the topmost die D or aroundthe sides of the dies A, B, C and D as fluid flow will generally takethe path of least resistance. One skilled in the art understands thatthe barriers 96 may have any configuration or shape so long as suchconfiguration or shape directs the fluid flow F substantially to regionsbetween the dies (e.g., to the region of the bumps 30) and substantiallyblocks fluid flow over the top of the topmost die D or around the sidesof the stacked dies A, B, C and D.

To further dissipate heat and enhance the cooling of cooling fluid 60,in another embodiment, a heat sink 70 is thermally coupled to conduit85. Heat sink 70 draws heat from cooling fluid 60 to ambient air therebycooling cooling fluid 60.

It is important to maintain the level of the cooling fluid 60 at aprescribed level as otherwise the resultant cooling may be insufficient.Accordingly, aspects of the present disclosure allow monitoring of thelevel of the cooling fluid 60 in a relatively easy manner, andconsequently reduce the risk of excessive temperature increases and theassociated problems. In one embodiment, the stacked die package 10 ofFIG. 3 includes temperature sensors 55A and 55B. Temperature sensors 55Aand 55B may be positioned at one or more places on an exterior wallportion of the housing 40 and therefore may be retrofitted in arelatively easy and inexpensive manner. According to one embodiment,temperature sensor 55A is fitted at or near the lower inlet or openingin housing 40, whereas temperature sensor 55B is positioned at or nearthe upper outlet or opening in housing 40 but at a level just below theminimum prescribed cooling fluid level. Temperature sensors 55A and 55Bmay be resistive temperature sensors that indicate a change intemperature by a change in electrical resistance. The measuredtemperature is of course dependent on the level of the cooling fluid 60,the ambient air temperature, and a given stacked die package design. Toread the measured outputs of the temperature sensors, the stack diepackage 10 may comprise a computer (not shown) having suitable computersoftware to receive the outputs and provide information indicative of anoperation property from the measured temperatures. In calculating thelevel of the cooling fluid 60 in the housing 40, the computer compares avalue associated with the measured outputs with predetermined valuesexpected for a normal operation condition. If a difference between theexpected predetermined outputs and measured outputs is above a thresholdvalue, the computer is arranged to generate an alarm condition. In otherembodiments, the computer may be arranged to generate a warning signalwhen the cooling fluid 60 drops below a prescribed minimum level.

Although in the above-described embodiments two temperature sensors areemployed, a person skilled in the art will appreciate that in furthervariations more than two temperature sensors may be used. Further, theteachings of the present disclosure of employing temperature sensors formonitoring the cooling fluid level in a stacked die package can beequally applied to a multi-chip system package, such as the one depictedin FIG. 2.

FIG. 4 is a flowchart illustrating a method 400 of manufacturing astacked die package having a cooling mechanism according to anembodiment of the present disclosure. At step 410 of method 400 asubstrate is provided. Next, at step 420 a first die is placed over thesubstrate. Then, at step 430 the first die is bonded to the substrate.As an example, the bonding of the first die to the substrate can beaccomplished via a flip chip bonding process.

At step 440 a second die is placed over the first die. Next, at step 450the second die is bonded to the first die. As an example, the bonding ofthe second die to the first die can be accomplished via a flip chipbonding process.

At step 460 is placed over the first and second dies such that thehousing seals the dies from the environment. Next, at step 470 a coolingfluid is added to immerse the first and second dies therein.

In some embodiments, step 420 to step 450 may be replaced with a step offirst bonding a first die to a second die, placing the bonded first andsecond dies over the substrate, and then bonding the dies to thesubstrate.

The teachings of the present disclosure protect dies in a stacked diepackage, individual chips, and/or components in a multi-chip systempackage from excessive heat that would otherwise compromise theperformance and/or reliability of the dies, chips and/or components inthese packages. It is another advantage that embodiments of the presentdisclosure require minimal modifications to the current design forexisting packages that are low cost and simple to implement. It is yetanother advantage of the present disclosure that underfill materials arenot needed between stacked dies (not including those dies that aredisposed on a substrate or an interposer) in contrast to conventionalstacked die packages or multi-chip packages. It is yet another advantagethat embodiments of the present disclosure provide monitoring of thelevel of a cooling fluid in a relatively easy manner and consequentlyreduce the risk of excessive temperature increases and the associatedproblems. It is contemplated that the cooling fluid systems and methodsof the present disclosure can be used in any electronic packagingsystem, such as stacked chip package, multi-chip package, or stackedchip and multi-chip package that require a cooling fluid for coolingand/or excess heat prevention.

In the preceding detailed description, the present invention isdescribed with reference to specifically exemplary embodiments thereof.It will, however, be evident that various modifications, structures,processes, and changes may be made thereto without departing from thebroader spirit and scope of the present disclosure. The specificationand drawings are, accordingly, to be regarded as illustrative and notrestrictive. It is understood that embodiments of the present disclosureare capable of using various other combinations and environments and arecapable of changes or modifications within the scope of the invention asexpressed herein.

What is claimed is:
 1. An apparatus for cooling a stacked die package,comprising: a first die above a substrate; a second die above the firstdie; a cooling fluid in fluid communication with the first die and thesecond die, the cooling fluid for absorbing thermal energy from thefirst and the second die; a housing containing the first and seconddies, the housing sealing the first and second dies from an environment,wherein the housing includes a first opening and a second opening, thefirst and second openings being vertically displaced from one another; aconduit having one end connected to the first opening and the other endconnected to the second opening, the conduit allowing the cooling liquidto circulate from the first opening to the second opening; a firsttemperature sensor being arranged to provide an output that is dependenton a local temperature at the first opening; and a second temperaturesensor being arranged to provide an output that is dependent on a localtemperature at the second opening, wherein the outputs of the first andsecond temperature sensors relative to each other are indicative of alevel of the cooling fluid.
 2. The apparatus of claim 1, wherein thefirst and second temperature sensors are positioned at exterior portionsof the housing.
 3. The apparatus of claim 1, wherein the second openingis higher than the first opening.
 4. The apparatus of claim 1, whereinthe second temperature sensor is located proximate to the second openingat a level just below a minimum prescribed cooling fluid level.
 5. Theapparatus of claim 1, wherein the apparatus is arranged so that awarning signal is initiated when a drop in the level of the coolingfluid below a predetermined threshold level is detected.
 6. Theapparatus of claim 1, wherein the first and the second temperaturesensors are resistive temperature sensors that indicate a change intemperature by a change in electrical resistance.
 7. The apparatus ofclaim 1, comprising a computer arranged to read the measured outputs ofthe temperature sensors and provide information indicative of anoperation property from the measured temperatures.
 8. The apparatus ofclaim 7, wherein the computer is arranged to compare the informationindicative of an operating property with information associated with anexpected predetermined operating property.
 9. The apparatus of claim 7,wherein the computer is arranged to calculate a value that is associatedwith a level of the cooling fluid.
 10. The apparatus of claim 1, furthercomprising a pump coupled to the conduit for pumping the cooling fluidfrom the first opening to the second opening.
 11. The apparatus of claim10, wherein the pump pumps the cooling fluid from a bottom region of thehousing to an upper region of the housing.
 12. An apparatus for coolinga multi-chip package system, comprising: a multi-chip package having twoor more dies; a cooling fluid in fluid communication with the two ormore dies, the cooling fluid for absorbing thermal energy from the twoor more dies; a housing containing the multi-chip package, the housingsealing the multi-chip package from the environment, wherein the housingincludes a first opening and a second opening, the first and secondopenings being vertically displaced from one another; a conduit havingone end connected to the first opening and the other end connected tothe second opening, the conduit allowing circulating the cooling liquidto circulate from the first opening to the second opening; a firsttemperature sensor being arranged to provide an output that is dependenton a local temperature at the first opening; and a second temperaturesensor being arranged to provide an output that is dependent on a localtemperature at the second opening, wherein the outputs of the first andsecond temperature sensors relative to each other are indicative of alevel of the cooling fluid.
 13. The apparatus of claim 12, wherein thefirst and second temperature sensors are positioned at exterior portionsof the housing.
 14. The apparatus of claim 12, wherein the secondopening is higher than the first opening.
 15. The apparatus of claim 12,wherein the second temperature sensor is located proximate the secondopening at a level just below a minimum prescribed cooling fluid level.16. The apparatus of claim 12, wherein the apparatus is arranged so thata warning signal is initiated when a drop in the level of the coolingfluid below a predetermined threshold level is detected.
 17. Theapparatus of claim 12, wherein the first and the second temperaturesensors are resistive temperature sensors that indicate a change intemperature by a change in electrical resistance.
 18. The apparatus ofclaim 12, comprising a computer arranged to read the measured outputs ofthe temperature sensors and provide information indicative of anoperation property from the measured temperatures.
 19. The apparatus ofclaim 18, wherein the computer is arranged to compare the informationindicative of an operating property with information associated with anexpected predetermined operating property.
 20. The apparatus of claim18, wherein the computer is arranged to calculate a value that isassociated with a level of the cooling fluid.